This invention relates to bearing assemblies. In particular, the present invention is a thermal compensation mechanism, that permits expansion and contraction, due to temperature variations, of a preloaded pair of duplex ball bearing mechanisms having components with different coefficients of thermal expansion.
In the aviation industry and the space industry, it is critical that aircraft and spacecraft systems be manufactured to be as lightweight and durable as possible. In particular with regard to spacecraft, it is essential that lightweight materials be used to manufacture spacecraft components of spacecraft systems, since each incremental increase in spacecraft component weight requires an incremental increase in the amount of rocket fuel required to launch the spacecraft into outer space. Because spacecraft components of spacecraft systems are submitted to high structural stresses and extreme temperature fluctuations within the range of 180.degree. F. to -65.degree. F., it is also essential that spacecraft components be manufactured of materials that are durable. However, it is not always possible to manufacture spacecraft components of a spacecraft system to a desired weight and a desired durability using a single material. Hence, spacecraft systems often are comprised of spacecraft components manufactured of various materials, with the choice of a particular material being dependent upon the purpose served by the particular spacecraft component. Typically, spacecraft components subjected to high structural stresses are manufactured from heavy, structurally strong and durable materials, while spacecraft components that are not subject to extreme structural stresses are manufactured from lightweight, less durable materials.
One concern of tailoring the material to the purpose of the spacecraft component is that different materials exhibit different physical properties. One of these physical properties that can vary from one material to the next is the coefficient of thermal expansion (CTE). For example, bearing mechanisms (such as preloaded, angular contact, duplex bearing assemblies) within spacecraft components typically use dissimilar materials for different bearing mechanism components. For instance, ceramic balls are used with steel rings (i.e., races) to form a hybrid bearing. The ceramic balls are lightweight, wear resistant and exhibit low rolling friction, while the steel rings are heavy and provide the necessary strength and durability for mounting to other spacecraft components.
The ceramic material of the bearing balls typically has a CTE of 1.6E-06 in./in. -.degree. F., while the steel material of the bearing rings has a much higher CTE of 5.6E-06 in./in. -.degree. F., resulting in a bearing mechanism that is very sensitive to operating temperature. For example, as the operating temperature of the hybrid bearing mechanism of this type decreases, the steel rings contract more rapidly than the ceramic balls, resulting in a decrease in the diametrical clearance between the balls and the rings. This results in an effective decrease in the bearing contact angle and an increase in the bearing preload, which causes unwanted increases in the drag torque and operating stress of the bearing mechanism. These drag torque and operating stress increases result in a bearing life reduction that is critical in a spacecraft environment.
Currently, this unlike expansion and contraction of bearing components, due to the affect of temperature variations on materials having different CTE's, is dealt with by designing the bearing mechanism so that it will have the desired contact angle and preload at the anticipated operating temperature and either limiting the temperature variations seen by the bearing mechanism or accepting the preload and contact angle changes and the consequent drag torque increase and life reduction that occurs due to the operating temperature range.
A similar problem, as described above in relation to a hybrid bearing, also exists due to the difference in CTE's between bearing mechanism materials and bearing mounting structure materials. Typically, in a spacecraft component, the steel rings of the bearing mechanism are secured to mounting structures which are made of aluminum (a lightweight material). The steel of the bearing mechanism rings has a CTE of 5.6E-06 in./in. -.degree. F., while the aluminum of the mounting structures has a much higher CTE of 12.4E-06 in./in. -.degree. F. In the environment of space, the mounting structures and bearing mechanism are subjected to temperature variations within the range of 180 degrees F. to -65 degrees F. These temperature variations, together with the differing CTE's of steel and aluminum, cause the mounting structures and bearing mechanism to expand and contract at different rates. in turn, this differential expansion or contraction causes gaps or contact pressure points, respectively, between the bearing mechanism and the mounting structures, which results in large bearing stresses and high drag torque and ultimately shorter bearing life.
There is a need for thermal compensation mechanisms for bearing assemblies. In particular, there is a need for a thermal compensation mechanism that will virtually eliminate bearing stresses due to the affects of extreme temperature fluctuations on the materials (having differing CTE's) of the bearing assembly components. In addition, the thermal compensation mechanism should be relatively maintenance free and provide a weight efficient structure.